Controller and control method for internal combustion engine

ABSTRACT

To provide a controller and control method for an internal combustion engine capable of estimating a discharge plasma length and the in-cylinder flow speed accurately by easy method. A controller and a control method for an internal combustion engine detects a secondary voltage which is a voltage generated by the secondary coil, calculates a minimum value of secondary voltage during a discharge period, calculates a discharge plasma length based on the secondary voltage and the minimum value of secondary voltage, and calculates an in-cylinder flow speed based on a time change of the discharge plasma length and a Coulomb force.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2017-124787 filed onJun. 27, 2017 including its specification, claims and drawings, isincorporated herein by reference in its entirety.

BACKGROUND

The present disclosure relates to a controller and a control method foran internal combustion engine that is provided with an ignition coil.

To date, as the ignition device of the internal combustion engine, therehas been known an ignition system in which the voltage stepped up by theignition coil is supplied to the ignition plug, the spark discharge(here, it means a dielectric breakdown and a subsequent formation ofdischarge plasma) is generated between the gap of the ignition plugdisposed in the combustion chamber of the internal combustion engine,and the spark ignition is performed to the fuel-air mixture in thecombustion chamber by energy which the spark discharge supplies.

In recent years, the demand to the ignition system becomes highlyfunctional for downsizing by supercharging, high compression ratio, andhigh dilution combustion which are the trends aiming to improve the fuelefficiency of the internal combustion engine. That is, in the internalcombustion engine downsized by supercharging, or the internal combustionengine of high compression ratio, there is the trend that the internalcylinder pressure at the time of spark ignition becomes highsignificantly, as compared with the conventional internal combustionengine; consequently, because breakdown voltage also becomes high,output energy increase of the ignition coil is required, and highwithstand voltage performance of the ignition coil and the ignition plugis also required. High dilution combustion is high EGR combustion andhigh lean burn combustion. Such fuel-air mixture generally has a narrowstable combustion region. In order to burn this stably, it is known thatit is effective to increase the output energy of the ignition coil, toextend the discharge period, to strengthen the in-cylinder flow, and thelike.

By the way, when performing spark ignition in the internal combustionengine which can generate a strong in-cylinder flow using the ignitioncoil which increased output energy as described above, it is known thatthe phenomena that the discharge plasma which occurs between the gap ofthe ignition plug is flowed and extends long by the in-cylinder flowwill occur. By being flowed and extending of the discharge plasma inthis way, rather, the fuel-air mixture around the discharge plasma isactivated, and the influence of cooling by the electrode also decreasesbecause the discharge plasma departs from the ignition plug; therefore,it is known that even in high dilution combustion, it is effective instabilization of combustion. This phenomenon is described in JP2008-88947 A, JP 4978737 B, and JP 2015-200257 A, for example.

In the technology disclosed in JP 2008-88947 A, by interruptingdischarge when the flow of discharge sparks is observed based on theignition current value, and suppressing the difference between the casewhere the discharge was flowed and the case where the discharge was notflowed, the output fluctuation between cycles is suppressed. In thetechnology disclosed in JP 4978737 B, discharge path length iscalculated based on discharge voltage, and the length of discharge iscontrolled by the electromagnet provided in the ignition plug. In themethod disclosed in JP 2015-200257 A, an air flow speed in thecombustion chamber is estimated, based on a change in slope of thesecondary current accompanying a change in the secondary voltage.

SUMMARY

The applicant of the present disclosure performed the visualizationexperiment of spark ignition uniquely in environment with flow andwithout flow, and measured the secondary current and the secondaryvoltage of the ignition coil at this time. As a result, when there is noflow, i.e., discharge is not flowed, it was found out that discharge isperformed in the state where the secondary voltage is almost constant,and the secondary current decreases gradually. When there is flow, i.e.,discharge is flowed, it was found out that the secondary voltageincreases as discharge plasma extends, and decrease of the secondarycurrent also becomes early, as compared with the case where there is noflow.

The applicant of the present disclosure advances study further andstudies the relationship among the secondary current, the secondaryvoltage, the discharge plasma length, the in-cylinder flow, and thecombustion stability; and the applicant considered that if the dischargeplasma length and the in-cylinder flow speed can be calculated moreaccurately, the combustion stability can be improved by operating thein-cylinder flow and the ignition energy based on the discharge plasmalength and the in-cylinder flow speed. This is because it is consideredthat since the discharge plasma is blown off when the in-cylinder flowis too strong and the discharge plasma does not extend when thein-cylinder flow is too weak, the in-cylinder flow of a degree that thedischarge plasma is not blown off is the optimal; and the dischargeplasma becomes difficult to be blown off by increasing the ignitionenergy.

However, the methods disclosed in JP 2008-88947 A, JP 4978737 B, and JP2015-200257 A only mention that there is some correlation among thesecondary current, the secondary voltage, the discharge plasma length,and the in-cylinder flow speed; and even if the relationship between thesecondary voltage and the discharge plasma length is memorized as acontrol map, it is unknown how detailed map is required; also, sincecontent such as an approximate expression is not shown, it is unknownhow discharge plasma length is calculated concretely. The same appliesto the relationship among the secondary current, the secondary voltage,and the in-cylinder flow speed. Although it is considered to use thesecondary voltage directly instead of the discharge plasma length andthe in-cylinder flow speed, since the secondary voltage changes largelyaccording to the environment in the cylinder (pressure, temperature,air-fuel ratio, and the like), it is considered that parameter settingand matching become complicated for controlling by the secondaryvoltage.

Thus, it is desirable to provide a controller and control method for aninternal combustion engine capable of estimating a discharge plasmalength and the in-cylinder flow speed accurately by easy method.

A controller for an internal combustion engine according to the presentdisclosure is a controller for an internal combustion engine that isprovided with an ignition plug which has a plug gap disposed in acombustion chamber, and an ignition coil which has a primary coil inwhich power is supplied from a direct current power source and asecondary coil which has more winding number than the primary coil andgenerates high voltage supplied to the ignition plug, the controller forthe internal combustion engine includes:

an ignition coil control unit that shuts down after connectingelectrically the primary coil and the direct current power source forgenerating high voltage in the secondary coil and generating sparkdischarge in the plug gap;

a secondary voltage detection unit that detects a secondary voltagewhich is a voltage generated by the secondary coil;

a secondary voltage minimum value calculation unit that calculates aminimum value of the secondary voltage during a discharge period basedon the detected secondary voltage;

a discharge plasma length calculation unit that calculates a length ofthe discharge plasma based on the secondary voltage and the minimumvalue of the secondary voltage; and

an in-cylinder flow calculation unit that calculates an in-cylinder flowspeed which is a flow speed of gas in the combustion chamber, based on atime change of the length of the discharge plasma and a Coulomb forceapplied to charged particles of the discharge plasma.

A control method for an internal combustion engine according to thepresent disclosure is a control method for an internal combustion enginethat is provided with an ignition plug which has a plug gap disposed ina combustion chamber, and an ignition coil which has a primary coil inwhich power is supplied from a direct current power source and asecondary coil which has more winding number than the primary coil andgenerates high voltage supplied to the ignition plug, the control methodfor the internal combustion engine includes:

an ignition coil control step that shuts down after connectingelectrically the primary coil and the direct current power source forgenerating high voltage in the secondary coil and generating sparkdischarge in the plug gap;

a secondary voltage detection step that detects a secondary voltagewhich is a voltage generated by the secondary coil; a secondary voltageminimum value calculation step that calculates a minimum value of thesecondary voltage during a discharge period based on the detectedsecondary voltage;

a discharge plasma length calculation step that calculates a length ofdischarge plasma based on the secondary voltage and the minimum value ofthe secondary voltage; and

an in-cylinder flow calculation step that calculates an in-cylinder flowspeed which is a flow speed of gas in the combustion chamber, based on atime change of the length of the discharge plasma and a Coulomb forceapplied to charged particles of the discharge plasma.

According to the controller and the control method for the internalcombustion engine concerning the present disclosure, by calculating theminimum value of the secondary voltage during the discharge period, thesecondary voltage just after discharge starting which is varied everyignition can be detected. Then, based on the minimum value of thesecondary voltage during the discharge period in addition to thesecondary voltage, the length of discharge plasma can be estimatedaccurately by easy method. Due to Coulomb force applied to chargedparticles of the discharge plasma, the discharge plasma shows adifferent behavior from the surrounding in-cylinder flow, and it doesnot become “the change speed of discharge plasma length=In-cylinder flowspeed” simply. Since the in-cylinder flow speed is calculated based onCoulomb force in addition to the time change of the discharge plasmalength, the in-cylinder flow speed can be estimated with good accuracyby easy method using the discharge plasma length.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic configuration diagram of an internal combustionengine and a controller according to Embodiment 1 of the presentdisclosure;

FIG. 2 is a schematic circuit diagram of ignition coil and spark plugaccording to Embodiment 1 of the present disclosure;

FIG. 3 is a block diagram of a controller according to Embodiment 1 ofthe present disclosure;

FIG. 4 is a hardware configuration diagram of a controller according toEmbodiment 1 of the present disclosure;

FIG. 5 is a timing chart showing the behavior of the secondary coil sidein the case where there is no extension of discharge plasma according toEmbodiment 1 of the present disclosure;

FIG. 6 is a timing chart showing the behavior of the secondary coil sidein the case where there is extension of discharge plasma according toEmbodiment 1 of the present disclosure;

FIG. 7 is an image figure showing discharge plasma in the case wherethere is no extension of discharge plasma according to Embodiment 1 ofthe present disclosure;

FIG. 8 is an image figure showing discharge plasma in the case wherethere is extension of discharge plasma according to Embodiment 1 of thepresent disclosure;

FIG. 9 is an image figure of discharge plasma extension in the concept(A) according to Embodiment 1 of the present disclosure;

FIG. 10 is an image figure of discharge plasma extension in the concept(B) according to Embodiment 1 of the present disclosure;

FIG. 11 is an image figure of discharge plasma extension in the concept(C) according to Embodiment 1 of the present disclosure;

FIG. 12 is an image figure of discharge plasma modeled in U shapeaccording to Embodiment 1 of the present disclosure;

FIG. 13 is an image figure for explaining Maxwell stress according toEmbodiment 1 of the present disclosure;

FIG. 14 is an image figure for explaining the equation of momentumaround of discharge plasma according to Embodiment 1 of the presentdisclosure;

FIG. 15 is a flowchart showing calculation processing of dischargeplasma length according to Embodiment 1 of the present disclosure;

FIG. 16 is a figure for explaining secondary voltage and the like whichare memorized in storage apparatus according to Embodiment 1 of thepresent disclosure;

FIG. 17 is a timing chart showing the behavior of the secondary coilside in the case where blow off of discharge plasma occurs according toEmbodiment 1 of the present disclosure;

FIG. 18 is a flowchart showing processing which operates in-cylinderflow based on discharge plasma length according to Embodiment 1 of thepresent disclosure;

FIG. 19 is a figure for explaining calculation of flow correlation valueaccording to discharge plasma length and blow off number according toEmbodiment 1 of the present disclosure;

FIG. 20 is a figure for explaining calculation of flow correlation valueaccording to in-cylinder flow speed and blow off number according toEmbodiment 1 of the present disclosure; and

FIG. 21 is a schematic circuit diagram of ignition coil and ignitionplug according to other Embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS Embodiment 1

A controller 50 for an internal combustion engine 1 (hereinafter,referred to simply as the controller 50) according to Embodiment 1 willbe explained with reference to the drawings. FIG. 1 is a schematicconfiguration diagram of the internal combustion engine 1 and thecontroller 50 according to Embodiment 1; FIG. 2 is a schematic circuitconfiguration diagram of an ignition plug 12, an ignition coil 13, andthe controller 50; FIG. 3 is a block diagram of the controller 50. Theinternal combustion engine 1 and the controller 50 are mounted in avehicle; the internal combustion engine 1 functions as a driving-forcesource for the vehicle (wheels).

1. The Configuration of the Internal Combustion Engine 1

The configuration of the internal combustion engine 1 will be explained.The internal combustion engine 1 has a combustion chamber 25 in which afuel-air mixture is combusted. The combustion chamber 25 is configuredby a cylinder and a piston. Hereinafter, “in the combustion chamber” isalso referred to “in the cylinder”. The internal combustion engine 1 isprovided with an intake path 23 for supplying air to the combustionchamber 25 and an exhaust path 14 for discharging exhaust gas from thecombustion chamber 25.

An air flow sensor 2 which outputs the electric signal according to aflow rate of the intake air taken into the intake path 23 fromatmospheric air is provided in the upstream side part of the intake path23. An electronic control type throttle valve 4 which opens and closesthe intake path 23 is provided in the part of the intake path 23 at thedownstream side of the air flow sensor 2. A throttle position sensor 3which outputs an electric signal according to the opening degree of thethrottle valve 4 is provided in the throttle valve 4. The part of theintake path 23 at the downstream side of the throttle valve 4 is anintake manifold 19. The upstream side part of the intake manifold 19 isa surge tank 5 for suppressing an intake air ripple, and the downstreamside part of the intake manifold 19 is an intake port 6.

The internal combustion engine 1 is provided with an EGR passage 21which recirculates the exhaust gas from the exhaust path 14 to theintake manifold 19, and an electronic control type EGR valve 15 whichopens and closes the EGR passage 21. In the intake manifold 19, thereare provided a manifold pressure sensor 7 which outputs an electricsignal according to a manifold pressure Pb, which is the pressure of gasin the intake manifold 12, and a manifold temperature sensor 8 whichoutputs an electric signal according to a manifold temperature, which isthe temperature of gas in the intake manifold 19.

The injector 9 which injects fuel into the combustion chamber 25 isprovided in the combustion chamber 25. The injector 9 may be provided inthe intake port 6 so as to inject fuel into the intake port 6.

On the top of the combustion chamber 25, there is provided an ignitionplug 12 for igniting a fuel-air mixture. An ignition coil 13 forsupplying ignition energy to the ignition plug 12 is provided. On thetop of the combustion chamber 25, there are provided an intake valve 10for adjusting the amount of intake air to be taken from the intake path23 into the combustion chamber 25 and an exhaust valve 11 for adjustingthe amount of exhaust gas to be exhausted from the combustion chamber 25to the exhaust path 14. The intake valve 10 is provided with an intakevariable valve timing mechanism which makes the opening and closingtiming thereof variable. The exhaust valve 11 is provided with anexhaust variable valve timing mechanism which makes the opening/closingtiming thereof variable. Each of the intake and exhaust variable valvetiming mechanisms 10, 11 has an electric actuator which changes a phaseangle of the opening and closing timing of the valve. On the crankshaftof the internal combustion engine 1, there is provided a rotary plate 16which has a plurality of teeth, and there is provided a crank anglesensor 17 which outputs an electric signal according to the rotation ofthe rotary plate 16.

<Spark Plug 12 and Ignition Coil 13>

FIG. 2 shows a circuit configuration diagram of the spark plug 12 andthe ignition coil 13. The spark plug 12 is provided with a plug gap 122which is disposed in the combustion chamber 25 and generates thedischarge plasma. The ignition plug 12 is provided with a resistance 121which is connected in series with the plug gap 122 and suppresses aradio noise.

The ignition coil 13 is provided with a primary coil 131 in which poweris supplied from a direct current power source 20, and a secondary coil132 which has more winding number than the primary coil 131 andgenerates the high voltage supplied to the ignition plug 12. The primarycoil 131 and the secondary coil 132 are wound around the common ironcore (core) 136. The primary coil 131, the secondary coil 132, and thecore 136 constitute a step-up transformer. The ignition coil 13 isprovided with a switching device as the igniter 133 which turns on orturns off the electrical connection from the direct current power source20 to the primary coil 131. The ignition coil 13 is provided with anignition coil voltage sensor 134 which outputs an electric signalaccording to a secondary voltage V2 which is a voltage generated by thesecondary coil 132. The ignition coil voltage sensor 134 is a voltagedividing circuit which divides the secondary voltage V2 by tworesistances connected in series, and is connected in parallel with theignition plug 12. The divided voltage of the connection point of tworesistances is inputted to the controller 50.

In the present embodiment, one end of the primary coil 131 is connectedto the positive electrode of the direct current power source 20, and theother end of the primary coil 131 is connected to the ground (thenegative electrode of the direct current power source 20) via theigniter 133. By controlling on/off the igniter 133 by the controller 50,the electrical connection from the direct current power source 20 to theprimary coil 131 is turned on or turned off. One end of the secondarycoil 132 is connected to the positive electrode of the direct currentpower source 20, and the other end of the secondary coil 132 isconnected to the ground via the ignition plug 12. The other end of thesecondary coil 132 is connected to the ground via the ignition coilvoltage sensor 134 which is the voltage dividing circuit. The controller50 is provided with a switching device as an igniter driving circuit 501which turns on or turns off the igniter 133. The igniter driving circuit501 is operated by a command signal from the computing processing unit90.

2. The Configuration of the Controller 50

Next, the controller 50 will be explained. The controller 50 is the onewhose control subject is the internal combustion engine 1. As shown inFIG. 3, the controller 50 is provided with control units such as anignition coil control unit 51, a secondary voltage detection unit 52, asecondary voltage minimum value calculation unit 53, a discharge plasmalength calculation unit 54, a flow correlation value calculation unit55, a flow control unit 56, an ignition energy increase unit 57, and anin-cylinder flow calculation unit 58. The respective control units 51through 58 and the like of the controller 50 are realized by processingcircuits included in the controller 50. Specifically, as shown in FIG.4, the controller 50 includes, as processing circuits, a computingprocessing unit (computer) 90 such as a CPU (Central Processing Unit),storage apparatuses 91 that exchange data with the computing processingunit 90, an input circuit 92 which inputs external signals to thecomputing processing unit 90, an output circuit 93 which outputs signalsfrom the computing processing unit 90 to the outside, and the like.

As the calculation processor 90, ASIC (Application Specific IntegratedCircuit), IC (Integrated Circuit), DSP (Digital Signal Processor), FPGA(Field Programmable Gate Array), various kinds of logical circuits,various kinds of signal processing circuits, and the like may beprovided. As the calculation processor 90, a plurality of the same typeones or the different type ones may be provided, and each processing maybe shared and executed. As the storage apparatuses 91, there areprovided a RAM (Random Access Memory) which can read data and write datafrom the calculation processor 90, a ROM (Read Only Memory) which canread data from the calculation processor 90, and the like. The inputcircuit 92 is connected with various kinds of sensors and switches andis provided with an A/D converter and the like for inputting outputsignals from the sensors and the switches to the calculation processor90. The output circuit 93 is connected with electric loads and isprovided with a driving circuit and the like for outputting a controlsignal from the computing processing unit 90.

In addition, the computing processing unit 90 runs software items(programs) stored in the storage apparatus 91 such as a ROM andcollaborates with other hardware devices in the controller 50, such asthe storage apparatus 91, the input circuit 92, and the output circuit93, so that the respective functions of the control units 51 through 58included in the controller 50 are realized. Setting data items such asmap data and determination value to be utilized in the control units 51through 58 are stored, as part of software items (programs), in thestorage apparatus 91 such as a ROM.

In the present embodiment, the input circuit 92 is connected with theair flow sensor 2, the throttle position sensor 3, the manifold pressuresensor 7, the manifold temperature sensor 8, the crank angle sensor 17,the atmospheric pressure sensor 18, the ignition coil voltage sensor134, an accelerator position sensor 26, and the like. The output circuit93 is connected with the throttle valve 4, the injector 9, the intakevariable valve timing mechanism 10, the exhaust variable valve timingmechanism 11, the ignition coil 13, the EGR valve 15, and the like. Thecontroller 50 is connected with various kinds of unillustrated sensors,switches, actuators, and the like.

The controller 50 detects various kinds of driving conditions of theinternal combustion engine 1 and the vehicle based on the output signalsof various kinds of sensors and the like. For example, the controller 50detects a rotational speed of the internal combustion engine and a crankangle based on the output signal of the crank angle sensor 17 and thelike. The controller 50 calculates an intake air amount of the internalcombustion engine, a charging efficiency, an EGR rate, and the like,based on the output signals of the air flow sensor 2, the manifoldpressure sensor 7, and the like.

As basic control, the controller 50 calculates a fuel injection amount,an ignition timing, and the like, based on the detected drivingconditions, and performs driving control of the injector 9, the ignitioncoil 13, and the like. The controller 50 calculates an output torque ofthe internal combustion engine 1 demanded by the driver, based on theoutput signal of the accelerator position sensor 26 and the like;calculates a target charging efficiency, a target EGR rate, and the likefor realizing the demanded output torque; and controls the openingdegree of the throttle valve 4, the opening degree of the EGR valve 15,and the phase angles of the intake and exhaust variable valve timingmechanisms 10, 11 so as to achieve the target charging efficiency, thetarget EGR rate, and the like.

2-1. Ignition Coil Control Unit 51

The ignition coil control unit 51 implements an ignition coil controlprocessing (an ignition coil control step) that shuts down afterconnecting electrically the primary coil 131 and the direct currentpower source 20 for generating high voltage in the secondary coil 132and generating spark discharge in the plug gap 122. The ignition coilcontrol unit 51 calculates an energizing time and an ignition timing (anignition crank angle) to the primary coil 131. The ignition coil controlunit 51 calculates a point of time earlier than the ignition timing bythe energizing time, as an energization start timing. Then, the ignitioncoil control unit 51 turns on the igniter 133 via the igniter drivingcircuit 501 at the energization start timing, and energizes the primarycoil 131. The ignition coil control unit 51 turns off the igniter 133via the igniter driving circuit 501 at the ignition timing, and shutsdown the energization of the primary coil 131.

The ignition coil control unit 51 may calculate the energizing timecorresponding to the present driving condition, such as the rotationalspeed and the charging efficiency, by referring to an energizing timemap in which the relationship between the driving condition, such as therotational speed and the charging efficiency, and the energizing time ispreliminarily set. Alternatively, the ignition coil control unit 51 maycalculate an ignition energy corresponding to the present drivingcondition, such as the rotational speed and the charging efficiency, byreferring to an ignition energy map in which the relationship betweenthe driving condition, such as the rotational speed and the chargingefficiency, and the ignition energy is preliminarily set; and calculatethe energizing time by using a relational equation between theenergizing time and the ignition energy.

The ignition coil control unit 51 may calculate the ignition timingcorresponding to the present driving condition, such as the rotationalspeed and the charging efficiency, by referring to an ignition timingmap in which the relationship between the driving condition, such as therotational speed and the charging efficiency, and the ignition timing ispreliminarily set. Alternatively, the ignition coil control unit 51 maycalculate the ignition timing by the feedback control which changes theignition timing so that the combustion gravity center positioncalculated based on the cylinder internal pressure, which was detectedby a pressure sensor or was estimated using crank angle detectioninformation, approaches a target crank angle.

<Behavior at the Time of Ignition>

The behavior at the time of ignition will be explained. The primarycurrent I1 which flows into the primary coil 131 increases graduallyafter start of energization to the primary coil 131. A magnetic energycorresponding to a magnitude of the primary current I1 is stored in thecore 136. Then, when the energization to the primary coil 131 is shutdown, the primary current I1 becomes zero, and by the magnetic energystored in the core 136, the voltage of the secondary coil 132 rises andthe voltage between the plug gap 122 rises. When the voltage between theplug gaps 122 exceeds a breakdown voltage between the plug gaps 122, aspark discharge occurs between the plug gaps 122. Here, the sparkdischarge means whole discharge phenomenon by dielectric breakdown, andglow discharge or arc discharge which occurs between the plug gaps 122after dielectric breakdown. Plasma which occurs as a discharge path ofglow discharge or arc discharge is called discharge plasma. The plug gap122 is conducted via the discharge plasma occurred by the sparkdischarge, secondary current I2 flows from the secondary coil 132, andthe fuel-air mixture in the combustion chamber 25 is ignited by energyreleased at the plug gap 122.

2-2. Calculation Principle of Discharge Plasma Length L

<Behavior of in-Cylinder Flow and Discharge Plasma>

The behavior of the secondary coil 132 side which operates in this way,and the behavior of the discharge plasma which occurs at the plug gap122 will be explained with reference to FIG. 5 through FIG. 8. FIG. 5and FIG. 6 are timing charts showing the behavior of the secondary coil132 side, and FIG. 7 and FIG. 8 are image figures showing the dischargeplasma which occurs at the plug gap 122. In each figure, the secondaryvoltage V2 and the secondary current I2 generate in the negative side;but the direction where absolute value becomes large is explained asincrease or rise, and the direction where absolute value becomes smallis explained as decrease or fall.

FIG. 5 shows the behavior of the secondary coil 132 side in the casewhere there is no in-cylinder flow and no extension of discharge plasma;as shown in FIG. 7, the discharge plasma only wavers slightly whileoccurring at the plug gap 122, and the basic length of discharge plasmais almost the same as the distance between the plug gaps 122. Theenergization to the primary coil 131 is shut down at the time t0 of FIG.5. By energization shutdown, the secondary voltage V2 raised to thebreakdown voltage Vbk, and the dielectric breakdown occurs. Thesecondary voltage V2 drops after the dielectric breakdown, and becomesan almost constant discharge maintaining voltage after the time t0. Thesecondary current I2 increases stepwise from 0 after the dielectricbreakdown at the time t0, then decreases by almost constant slope, andbecomes zero at the time t2. This is because the magnetic energy storedin the core 136 falls gradually by release of the secondary current I2,thereby the secondary current I2 also falls gradually.

Next, FIG. 6 shows the behavior of the secondary coil 132 side in thecase where there is in-cylinder flow and extension of discharge plasma;as shown in FIG. 8, the discharge plasma extends gradually by thein-cylinder flow after occurring at the plug gap 122. At the time t0 ofFIG. 6, the energization to the primary coil 131 is shut down; by thisenergization shutdown, the secondary voltage V2 raised to the breakdownvoltage Vbk, and the dielectric breakdown occurs. The secondary voltageV2 once falls to a voltage of the same degree as the case of no flowafter the dielectric breakdown, and then increases in accordance withthe extension of discharge plasma. The secondary current I2 decreasesearlier than the case of no flow, and becomes 0 at the time t2* earlierthan the time t2 in the case of no flow. This is because by the rise ofthe secondary voltage V2, the release amount per unit time of themagnetic energy stored in the core 136 increases, and decrease of themagnetic energy becomes early. Since the release rate [J/s] of energy ispower consumption W=V2×I2, even when the secondary current I2 is thesame, when the secondary voltage V2 is large, release of the magneticenergy becomes fast.

<Calculation Principle of Discharge Plasma Length>

Based on the discharge phenomenon which occurs at the plug gap 122 andthe operation of the ignition coil 13 explained above, concept ofcalculation method of the discharge plasma length will be explained.When a resistance between the plug gaps 122 (referred to as a gapresistance) during discharge is set to Rg, the relationship of the nextequation is established among the secondary voltage V2, the secondarycurrent I2, and the gap resistance Rg.V2=I2·Rg  (1)

When a length of the discharge plasma (referred to as a discharge plasmalength) along a flow of discharge current is set to L, a cross-sectionarea of the discharge plasma cut by a plane which is perpendicular tothe flow of discharge current is set to S, and the discharge plasma issupposed to be a conductor of an electric conductivity σ, the gapresistance Rg can be expressed by the next equation.

$\begin{matrix}{{Rg} = \frac{L}{\sigma \cdot S}} & (2)\end{matrix}$

When the equation (2) is substituted in the equation (1) and modified,the relationship of the next equation is obtained.

$\begin{matrix}{{V\; 2} = {{I\;{2 \cdot \frac{L}{\sigma \cdot S}}} = {\frac{I\; 2}{\sigma \cdot S} \cdot L}}} & (3)\end{matrix}$<In the Case of No Flow>

The case of no flow (no extension of the discharge plasma) will beconsidered. As explained using FIG. 5 and FIG. 7, in the state of noflow, the discharge plasma length L is almost constant, and in thiscase, the secondary voltage V2 also becomes almost constant. Therefore,from the equation (3), the relationship of the next equation isestablished.

$\begin{matrix}{\frac{I\; 2}{\sigma \cdot S} = {\left. {{Const}.}\Rightarrow \right.\therefore{{I\; 2} \propto {\sigma \cdot S}}}} & (4)\end{matrix}$

Here, “Const.” denotes that it is a constant value. “σ·S” is aconductance per unit length of the discharge plasma, i.e., a floweasiness of current per unit length, the equation (4) shows that if thesecondary current I2 becomes small, current becomes difficult to flow.Its reciprocal 1/(σ·S) means a resistance per unit length.

Meanwhile, plasma is in the state where molecules constituting gas areionized and positive ions and electrons are moving separately; sincethis ionized gas contains charged particles, it shows conductivity. Therate of ionization of gas is called ionization degree η, and since it isconsidered that conductivity changes according to the ionization degreeη, it is considered that the ionization degree η and the electricconductivity σ are correlated. Since it is considered that theionization degree η and luminescence intensity of plasma are correlated,it is considered that the luminosity of discharge and the electricconductivity σ are also correlated. When the discharge plasma in thecase of no flow (no extension of discharge) was observed, it was foundout that the discharge plasma which was bright and thick just afterdischarge starting becomes dark and thin gradually as the secondarycurrent I2 decreases, and it disappears at the time of discharge end. Asa result of this, as described above, when the secondary current I2becomes small, it is assumed to be observed that σ·S which is a productof the electric conductivity σ and the cross-section area S also becomessmall.

<In the Case where there is Flow (Concept (A))>

Next, the case where there is flow will be considered. Here, as aconcept (A), the equation (4) considered in the case of no flow isapplied also to the case where there is flow. FIG. 9 shows an image ofthe discharge plasma just after discharge starting and the dischargeplasma during discharge plasma extension. Here, I2, σ, S, L just afterdischarge starting are expressed by I20, σ0, S0, L0 to which 0 wasattached at those ends, respectively; and I2, σ, S, and L duringdischarge plasma extension (during discharge) are expressed by I21, σ1,S1, L1 to which 1 was attached at those ends, respectively. SinceI2/(σ·S) is constant regardless of the discharge plasma length Laccording to the equation (4), as shown in the next equation,I20/(σ0·S0) just after discharge starting and I21/(σ1·S1) duringdischarge plasma extension become equal.

$\begin{matrix}{\frac{I\; 20}{\sigma\;{0 \cdot S}\; 0} = \frac{I\; 21}{\sigma\;{1 \cdot S}\; 1}} & (5)\end{matrix}$

According to the equation (3), the secondary voltage V20 just afterdischarge starting and the secondary voltage V21 during discharge plasmaextension can be expressed by the equation (6) and the equation (7),respectively.

$\begin{matrix}{{V\; 20} = {{I\;{20 \cdot \frac{L\; 0}{\sigma\;{0 \cdot S}\; 0}}} = {{\frac{I\; 20}{\sigma\;{0 \cdot S}\; 0} \cdot L}\; 0}}} & (6) \\{{V\; 21} = {{I\;{21 \cdot \frac{L\; 1}{\sigma\;{1 \cdot S}\; 1}}} = {{\frac{I\; 21}{\sigma\;{1 \cdot S}\; 1} \cdot L}\; 1}}} & (7)\end{matrix}$

When the equation (6) and the equation (7) are substituted in theequation (5) and rearranged, the next equation is derived.

$\begin{matrix}{\frac{V\; 20}{L0} = {{\left. \frac{V\; 21}{L\; 1}\Rightarrow \right.\therefore{L\; 1}} = {L\;{0 \cdot \frac{V\; 21}{V\; 20}}}}} & (8)\end{matrix}$

If this concept is right, the discharge plasma length L1 duringdischarge plasma extension can be calculated by the equation (8). Here,it can be assumed that the discharge plasma length L0 just afterdischarge starting is equal to the length Lg between the plug gaps 122.From the behavior of the secondary voltage V2 explained using FIG. 6,the minimum value of the secondary voltage V2 during a predeterminedperiod just after discharge starting can be used as the secondaryvoltage V20 just after discharge starting. Here, the minimum value ofthe secondary voltage V2 is measured because change of the secondaryvoltage V2 has some delay.

<In the Case where there is Flow (Concept (B))>

In the concept (A), it was considered that I2/σ·S is constant as theequation (5) regardless of change of the discharge plasma length L; butin the case where the discharge plasma length L changes every moment, itis hardly considered that I2/σ·S is always constant. Since flowing andextending of the discharge plasma by flow between the plug gaps 122occurs in a short time, it is also considered that the amount of theionized gas is the same and the discharge plasma extends by changingonly those positions. FIG. 10 shows an image of the discharge plasmabefore and after a very short time during discharge plasma extension.Here, I2, σ, S, L at the time t1 during discharge plasma extension areexpressed by I21, σ1, S1, L1, respectively; and I2, σ, S, L before thevery short time from the time t1 during discharge plasma extension areexpressed by I21*, σ1*, S1*, L1*, respectively.

It is assumed that the volume of discharge plasma is constant betweenbefore and after the very short time during discharge plasma extension,and it is considered that the next equation is established.S1*·L1*=S1·L1  (9)

Since a change that σ1·S1 decreases during the discharge plasmaextension, and a change that the secondary current I21 decreases do notcoincide with each other, it is considered that the relationship likethe equation (5) is not established. Since the change that the secondarycurrent I21 decreases is caused by the decrease of the magnetic energyof the core 136, it is considered that it is later than the change thatthe cross-section area 51 decreases. Therefore, it is assumed that thenext equation in which the secondary current I21 was corrected by thecross-section area before and after the very short time S1*, S1 isestablished.

$\begin{matrix}{\frac{I\; 21^{*}}{{{\sigma 1}^{*} \cdot S}\; 1^{*}} = {{\frac{I\; 21}{{{\sigma 1} \cdot S}\; 1} \cdot \frac{S\; 1}{S\; 1^{*}}} = {\frac{I\; 21}{{{\sigma 1} \cdot S}\; 1} \cdot \frac{L\; 1^{*}}{L\; 1}}}} & (10)\end{matrix}$

Although it was assumed that the equation (10) is established at beforeand after the very short time, when before the very short time isreplaced to the time t0 just after discharge starting, the next equationwhich replaced “1*” in the equation (10) to “0” is obtained.

$\begin{matrix}{\mspace{79mu}{{\frac{I\; 20}{\sigma\;{0 \cdot S}\; 0} = {{\frac{I\; 21}{{{\sigma 1} \cdot S}\; 1} \cdot \frac{S\; 1}{S\; 0}} = {\frac{I\; 21}{{{\sigma 1} \cdot S}\; 1} \cdot \frac{L\; 0}{L\; 1}}}}{\frac{V\; 20}{L0} = {\left. {\frac{V\; 21}{L\; 1} \cdot \frac{L\; 0}{L\; 1}}\Rightarrow{L\; 1^{2}} \right. = {\left. {L\;{0^{2} \cdot \frac{V\; 21}{V\; 20}}}\Rightarrow{\therefore{L\; 1}} \right. = {L\;{0 \cdot \sqrt{\frac{V\; 21}{V\; 20}}}}}}}}} & (12)\end{matrix}$

In this case, from the equation (6), the equation (7), and the equation(11), the relationship between the discharge plasma length L and thesecondary voltage V2 is obtained as the next equation.

$\begin{matrix}{\mspace{79mu}{{\frac{I\; 20}{\sigma\;{0 \cdot S}\; 0} = {{\frac{I\; 21}{\sigma\;{1 \cdot S}\; 1} \cdot \frac{S\; 1}{S\; 0}} = {\frac{I\; 21}{\sigma\;{1 \cdot S}\; 1} \cdot \frac{L\; 0}{L\; 1}}}}{\frac{V\; 20}{L\; 0} = {\left. {\frac{V\; 21}{L\; 1} \cdot \frac{L\; 0}{L\; 1}}\Rightarrow{L\; 1^{2}} \right. = {{\left. {L\;{0^{2} \cdot \frac{V\; 21}{V\; 20}}}\Rightarrow \right.\therefore{L\; 1}} = {L\;{0 \cdot \sqrt{\frac{V\; 21}{V\; 20}}}}}}}}} & (12)\end{matrix}$<In the Case where there is Flow (Concept (C))>

When a visualized observation of the discharge plasma in the case wherethere is flow (there is extension of the discharge plasma) wasperformed, even though the discharge plasma was extended, it did notbecome thin as it was extended like the concept (B), but it seemed thatthe discharge plasma was rather extended like the concept (A) withoutchanging the thickness of discharge plasma much. However, if it isconsidered that there is no change in the amount of ionized gas, it canbe thought that the concept (B) is right. Accordingly, although thedischarge plasma is extended like the concept (B), since it tries toreturn to a cross-section area of the discharge plasma corresponding tothe secondary current I2 by newly ionizing gas, it can be consideredthat the discharge plasma seems to be extended while thickness is thesame like the concept (A). FIG. 11 shows an image before and after thevery short time during discharge plasma extension in the case of thisconsideration. In this case, it is considered that current flows moreeasily than the concept (A) by the newly ionized gas. The above is aconcept (C).

Here, the next equation is obtained by generalizing the equation (8) andthe equation (12) which are the relational equations between thesecondary voltage and the discharge plasma length derived in the concept(A) and (B).

$\begin{matrix}{{L\; 1} = {L\;{0 \cdot \left( \frac{V\; 21}{V\; 20} \right)^{n}}}} & (13)\end{matrix}$

In the case of n=1, the equation (13) represents the equation (8) of theconcept (A), and this case is a concept that current flows as the sameas the case where the discharge plasma does not extend, and theresistance per unit length does not change. In the case of n=1/2, theequation (13) represents the equation (12) of the concept (B), and thiscase is a concept that since the discharge plasma becomes thin,resistance becomes large and current becomes difficult to flow.

Since the concept (C) is a concept that current becomes easy to flowrather than the concept (A), by the newly ionized gas when the dischargeplasma extends, if current flows rather than the concept (A), n>1 isexpected. It is calculated how much value the exponent n becomesconcretely in this concept, based on observation data. The exponent n iscalculated by the next equation, by using the minimum value of thesecondary voltage during the predetermined period just after thedielectric breakdown as the secondary voltage V20 just after dischargestarting, using the plug gap length Lg as the discharge plasma length L0just after discharge starting, and using the discharge plasma length Land the secondary voltage V2 at a certain time during discharge plasmaextension as the discharge plasma length L1 and the secondary voltageV21 during discharge plasma extension.

$\begin{matrix}{{\ln\left( {L\; 1} \right)} = {\left. {\ln\left\{ {L\;{0 \cdot \left( \frac{V\; 21}{V\; 20} \right)^{n}}} \right\}}\Rightarrow{\ln\left( {L\; 1} \right)} \right. = {{\ln\left( {L\; 0} \right)} + {n \cdot {\ln\left( \frac{V\; 21}{V\; 20} \right)}}}}} & (14) \\{{\therefore n} = \frac{{\ln\left( {L\; 1} \right)} - {\ln\left( {L\; 0} \right)}}{\ln\left( \frac{V\; 21}{V\; 20} \right)}} & \;\end{matrix}$

When the value of exponent n was calculated from the test result whichthe applicant of the present disclosure performed, it became a valuewithin a range of substantially 1.0 to 3.0. Since it is considered thatthis value of exponent n depends on the driving condition of theinternal combustion engine 1, especially the cylinder internal pressureand the inner cylinder temperature at the ignition timing, the value ofexponent n may be changed according to the cylinder internal pressure,or as the simplest approximation, considering the computation load ofthe controller 50, it may be simplified to n=2.

Summarizing the above, the discharge plasma length L1 at a certain timeduring discharge can be calculated by the equation (13), by setting thesecondary voltage at the time to V21, using the minimum value of thesecondary voltage during the predetermined period just after thedielectric breakdown as the secondary voltage V20 just after dischargestarting, and using the plug gap length Lg as the discharge plasmalength L0 just after discharge starting; and as the value of exponent nin this case, a value within a range of 1.0 to 3.0 may be used. In thecase where there is no flow and the discharge plasma does not extend,since the secondary voltage V2 is constant and the discharge plasmalength L is constant, the equation (13) is established regardless of thevalue of exponent n.

2-3. Calculation Principle of in-Cylinder Flow Speed Va

<Behavior of in-Cylinder Flow and Discharge Plasma>

Next, the calculation principle of an in-cylinder flow speed va will beexplained. It is considered that a flow speed of the gas near theignition plug can be calculated by considering a time change of thedischarge plasma length L. However, since the plasma is in the statewhere molecules constituting gas are ionized, separated into positiveions and electrons, and moving, charged particles having an electriccharge of a reverse sign are gathered around this electric charge byreceiving Coulomb force in the plasma. Accordingly, the plasma in a flowfield exhibits a behavior different from surrounding gas. As a result,since it does not become “the change speed of the discharge plasmalength=the in-cylinder flow speed” simply, it is necessary to derive arelationship between the change speed of the discharge plasma length andthe in-cylinder flow speed.

Then, Maxwell stress acting on an electric tube of force will be appliedas a gathering of Coulomb force acting in the discharge plasma; arelationship among the in-cylinder flow speed, the moving speed of theintermediate part of discharge plasma, and Maxwell stress will bemodeled; and a relational equation for estimating the in-cylinder flowspeed va will be derived. The details will be explained below.

FIG. 12 is an image figure which simply modeled that an extendeddischarge plasma becomes a U shape. The direction of the in-cylinderflow in the plug gap 122 is supposed to be a direction vertical to thelongitudinal direction of the plug gap 122. It is supposed that thedischarge plasma of U shape consists of a first side part 30 extendingin the in-cylinder flow direction from a center electrode 122 a of theplug gap 122, a second side part 31 extending in the in-cylinder flowdirection from an earth electrode 122 b of the plug gap 122, and abottom 32 connecting between a tip part of the first side part 30 in thein-cylinder flow direction and a tip part of the second side part 31 inthe in-cylinder flow direction. It is supposed that the length of thefirst side part 30 and the second side part 31 extends from 0 by thein-cylinder flow. It is supposed that the bottom 32 has the length ofthe plug gap length Lg, and corresponds to the intermediate part 32 ofdischarge plasma.

When the moving speed at which the intermediate part 32 of dischargeplasma moves in the in-cylinder flow direction is set to vp, and theelapsed time after discharge starting is set to t, the discharge plasmalength L can be expressed by the next equation. The extending speed ofthe first and the second side parts 30, 31 becomes equal to the movingspeed vp of the intermediate part 32 of discharge plasma. Since twoparts of the first side part 30 and the second side part 31 of the Ushape extend at the moving speed vp, extension of the discharge plasmalength L becomes twice of vp·t.L=Lg+2·vp·t  (15)

By the way, it is considered that the moving speed vp of theintermediate part 32 of discharge plasma changes over time. That is, itis considered that the moving speed vp≈0 just after discharge starting.When the force acting on the discharge plasma by the in-cylinder flow issufficiently larger than the Coulomb force acted in discharge plasma, itis considered that the moving speed vp≈the in-cylinder flow speed va,after time passes. Then, assuming that the moving speed vp of theintermediate part 32 of discharge plasma changes at each time point, thecalculation equation of the discharge plasma length L is expressed bythe discretized next equation. Here, Δt is a discretized time interval.m is a number of discretized time series data, and increases by 1 from 0just after discharge starting whenever the time interval Δt passes.Since two parts of the first side part 30 and the second side part 31 ofthe U shape extend at the moving speed vp even in the equation (16), theextension width of the discharge plasma length L during the timeinterval Δt becomes twice of 2·vp·Δt.L(m)=L(m−1)+2·vp(m)·ΔtL(0)=LgΔt=t(m)−t(m−1)  (16)

By the equation (16), a relationship between the moving speed vp (m) ofthe intermediate part 32 of the discharge plasma changing every momentand the discharge plasma length L (m) can be formulized. The equation(17) is obtained by solving the equation (16) for vp (m). From theequation (17), the moving speed vp (m) of the intermediate part 32 ofdischarge plasma can be calculated by time change of the dischargeplasma length L (m).

$\begin{matrix}{{{{vp}(m)} = {\frac{1}{2} \cdot \frac{{L(m)} - {L\left( {m - 1} \right)}}{\Delta\; t}}}{{{vp}(0)} = 0}{{L(0)} = {Lg}}} & (17)\end{matrix}$

Next, the Coulomb force acted in the discharge plasma will beconsidered. Since the plasma state is maintained by energy supply bydischarge, and current for discharge is flowing, it is considered thatthe discharge plasma is in the state where electric field is given fromoutside and also in the state where electric field is generated insideby moving of electrons. Based on this consideration, it can beconsidered that discharge plasma is the electric tube of force to whichthe electric field is applied and which floats in space, and it is knownthat Maxwell stress as a gathering of Coulomb force is applied to thiselectric tube of force. Maxwell stress F is expressed by the nextequation using Maxwell stress tensor T.

$\begin{matrix}{\mspace{79mu}{F = {\int{T \cdot {ndS}}}}} & (18) \\\left( {{{\because T} = {ɛ\; 0\begin{pmatrix}{E_{x}^{2} - {\frac{1}{2}E^{2}}} & {E_{x}E_{y}} & {E_{x}E_{z}} \\{E_{y}E_{x}} & {E_{y}^{2} - {\frac{1}{2}E^{2}}} & {E_{y}E_{z}} \\{E_{z}E_{x}} & {E_{z}E_{y}} & {E_{z}^{2} - {\frac{1}{2}E^{2}}}\end{pmatrix}}},{n = \begin{pmatrix}n_{x} \\n_{y} \\n_{z}\end{pmatrix}},{F = \begin{pmatrix}F_{x} \\F_{y} \\F_{z}\end{pmatrix}}} \right) & \;\end{matrix}$

Here, ε0 is a dielectric constant of vacuum, E is an electric field, nis a unit normal vector on a minute area dS, and subscripts x, y, z meaneach component of a Cartesian coordinate system. Although only Maxwellstress by electric field is considered here, Maxwell stress by magneticfield may be considered.

With reference to an image figure shown in FIG. 13 for explainingMaxwell stress applied to the electric tube of force, a force F1 appliedin a direction of the electric field and a force F2 applied in aperpendicular direction to the electric field will be explained. It isknown that the force F1 applied in the direction of the electric fieldis expressed by the next equation based on the equation (18), byconsidering that a vector n1 showing a perpendicular direction of aminute area dS1 is the same direction as the electric field.

$\begin{matrix}{\frac{{dF}\; 1}{{dS}\; 1} = {{\frac{1}{2} \cdot ɛ}\;{0 \cdot E^{2} \cdot n}\; 1}} & (19)\end{matrix}$

Similarly, it is known that the force F2 applied in the perpendiculardirection to the electric field is expressed by the next equation, byconsidering that a vector n2 showing a perpendicular direction of aminute area d52 is the perpendicular direction to the electric field.

$\begin{matrix}{\frac{{dF}\; 2}{{dS}\; 2} = {{{- \frac{1}{2}} \cdot ɛ}\;{0 \cdot E^{2} \cdot n}\; 2}} & (20)\end{matrix}$

Thus, it is found that attracting force is acting in the direction ofthe electric field, and repulsion force is acting in the perpendiculardirection of the electric field. That is, forces as if an elastic bodylike rubber exists are applied to the electric tube of force. From theequation (19) and the equation (20), magnitudes of the forces F1, F2 perunit area are the same, although directions of the forces are different.

Based on the above, a magnitude pm of Maxwell stress applied to thedischarge plasma is calculated. Although only the external electricfield is considered here for simplification, the internal electric fieldmay be considered for correction. A magnitude E (m) of the electricfield becomes a value obtained by dividing the secondary voltage V2applied to the plug gap 122 by the discharge plasma length L, as shownin the next equation.

$\begin{matrix}{{E(m)} = \frac{V\; 2(m)}{L(m)}} & (21)\end{matrix}$

The magnitude pm (m) of Maxwell stress is expressed by the nextequation. To the electric tube of force, the tension of pm (m) shrinkingthe electric tube of force in the direction along the electric tube offorce is applied, and the pressure of −pm (m) pressing the side face ofthe electric tube of force in the direction perpendicular to theelectric tube of force is applied. The electric tube of force behaveslike an elastic body which received electric strain.

$\begin{matrix}{{{pm}(m)} = {{\frac{1}{2} \cdot ɛ}\;{0 \cdot {E(m)}^{2}}}} & (22)\end{matrix}$

Next, FIG. 14 shows an image figure for explaining the equation ofmomentum around of the discharge plasma. Conservation of momentumbetween surrounding gas and the intermediate part 32 of discharge plasmais considered. By setting a radius of the discharge plasma to r, settinga pressure of the surrounding gas to p, and setting a density of thesurrounding gas to p, the next equation is established in a boundaryregion between the surrounding gas at the flow direction upstream sideof the intermediate part 32 of discharge plasma and the intermediatepart 32 of discharge plasma.ρ·A1·vp ² −ρ·A1·va ² =p·A1−(p·A1−pm·A1+2·pm·A2)A1=2·r·LgA2=π·r ²  (23)

Here, A1 is a project area of the intermediate part 32 of the dischargeplasma viewed in the flow direction, and A2 is a cross-section area ofthe intermediate part 32 of discharge plasma. The left-hand side of theequation (23) considers momentum, and the right-hand side considersforce acting on system. In the left-hand side, ρ·A1·vp² represents amomentum of the discharge plasma in the boundary, and ρ·A1·va²represents a momentum of the surrounding gas in the boundary. In theright-hand side, p·A1 of the first term represents a gas pressure actingon the discharge plasma from the upstream side surrounding gas in theboundary, p·A1 of the second term represents a gas pressure acting onthe upstream side surrounding gas from the discharge plasma in theboundary, and −pm·A1 of the third term represents the Maxwell stress(pressure) acting on the discharge plasma in the boundary. 2·pm·A2 ofthe fourth term in the right-hand side represent the Maxwell stress(tension) acting on both ends of the intermediate part 32 of dischargeplasma, and is doubled because of acting on the both ends.

The next equation is obtained by solving the equation (23) for va. Thein-cylinder flow speed va can be calculated by using the equation (17)for calculation of the moving speed vp of the intermediate part 32 ofdischarge plasma, and using the equation (22) for calculation of Maxwellstress pm. The gas density p is calculated by dividing the intake airamount taken into the combustion chamber 25 by a volume of thecombustion chamber 25 determined according to the crank angle. Here, acalculation coefficient K is a coefficient determined by the plug gaplength Lg and the radius r of the discharge plasma.

$\begin{matrix}{{{va} = {\sqrt{{vp}^{2} + \frac{{{{- {pm}} \cdot A}\; 1} + {{2 \cdot {pm} \cdot A}\; 2}}{{\rho \cdot A}\; 1}}\; = \sqrt{{vp}^{2} + {K \cdot \frac{pm}{\rho}}}}}{K = {\frac{\pi \cdot r}{Lg} - 1}}} & (24)\end{matrix}$

An approximate expression of the equation (24) become the next equation.As seen from this approximate expression, the in-cylinder flow speed vacan be calculated by adding a value according to Maxwell stress pm tothe moving speed vp of the intermediate part 32 of discharge plasma.

$\begin{matrix}{{va} = {\sqrt{{vp}^{2} + {K \cdot \frac{pm}{\rho}}} \cong {{vp} + {\frac{1}{2} \cdot \frac{1}{vp} \cdot K \cdot \frac{pm}{\rho}}}}} & (25)\end{matrix}$2-4. Configuration of Each Control Unit of Discharge Plasma LengthCalculation Unit 54 and in-Cylinder Flow Calculation Unit 58

Processings of each control unit configured based on the concepts ofcalculation method of the discharge plasma length and calculation methodof the in-cylinder flow speed explained above will be concretelyexplained with reference to the flowchart in FIG. 15.

<Secondary Voltage Detection Unit 52>

In the step 1201, the secondary voltage detection unit 52 performs asecondary voltage detection processing (a secondary voltage detectionstep) that detects the secondary voltage V2 which is the voltagegenerated by the secondary coil 132. In the present embodiment, thesecondary voltage detection unit 52 detects the secondary voltage V2based on the output signal of the ignition coil voltage sensor 134.

Specifically, the secondary voltage detection unit 52 performs an A/Dconversion of the divided voltage of the voltage dividing circuit as theignition coil voltage sensor 134 using the A/D converter of the inputcircuit 92, and detects the secondary voltage V2 based on an A/Dconversion value and a voltage dividing resistance ratio. The secondaryvoltage detection unit 52 performs the A/D conversion and detects thesecondary voltage V2 continuously at least from a shutdown time at whichthe ignition coil control unit 51 shut down the energization to theprimary coil 131 (time t0 of FIG. 5 and FIG. 6) to an end time ofdischarge (the time t2 of FIG. 5 and time t2* of FIG. 6).

For example, the secondary voltage detection unit 52 starts the A/Dconversion 100 μs before the energization shutdown time to the primarycoil 131, and performs the A/D conversion continuously every 50 ρs.Since the normal discharge period t0 to t2 is about 1 ms to 3 ms, thesecondary voltage detection unit 52 determines to sample a littlelonger, and performs the A/D conversion and detects the secondaryvoltage V2 for 4 ms after A/D conversion starting. Then, as shown inFIG. 17, the secondary voltage detection unit 52 stores the eachdetection value of the secondary voltage V2 to the storage apparatus 91such as RAM by correlating with a sampling number m. Whenever the A/Dconversion is performed, the sampling number m is increased by 1 from 0.

<Secondary Voltage Minimum Value Calculation Unit 53>

In the successive step 1202, the secondary voltage minimum valuecalculation unit 53 performs a secondary voltage minimum valuecalculation processing (a secondary voltage minimum value calculationstep) that calculates a minimum value V2 min of the secondary voltageduring the discharge period based on the secondary voltage V2 detectedby the secondary voltage detection unit 52. In the present disclosure,the secondary voltage V2 means an absolute value of the secondaryvoltage V2.

As seen from the equation (6) and the like, the secondary voltage V20just after discharge starting changes according to the plug gap lengthLg which becomes equal to the discharge plasma length L0 just afterdischarge starting, and the electric conductivity σ0 which changesaccording to the gas density (the cylinder internal pressure) just afterdischarge starting. According to this configuration, by calculating theminimum value V2 min of the secondary voltage during the dischargeperiod, the secondary voltage V20 just after discharge starting which isvaried every ignition can be accurately detected.

For example, the secondary voltage minimum value calculation unit 53changes the sampling number m from the A/D conversion start number tothe end number, and repeatedly performs a processing, as shown in thenext equation, which picks up a minimum value between the secondaryvoltage V2 (m) of the sampling number m and the minimum value V2 min ofthe secondary voltage in the previous processing and updates the pickedup value as the minimum value V2 min of the secondary voltage in thistime processing; and consequently, calculates the minimum value V2 minof the secondary voltage during the discharge period. Here, min( )represents the processing which picks up the minimum value. Thesecondary voltage V2 near 0 [V] which is out of the discharge period isexcluded from the minimum value V2 min of the secondary voltage.V2min=min(V2(m),V2min)  (26)

Processings after the step 1202 are performed by an interrupt processingat a predetermined crank angle after completion of the continuous A/Dconversion, for example, processings are performed in an interruptprocessing of BTDC75 degCA which comes first after completion of the A/Dconversion.

<Discharge Plasma Length Calculation Unit 54>

In the successive step 1203, the discharge plasma length calculationunit 54 performs a discharge plasma length calculation processing (adischarge plasma length calculation step) that calculates the dischargeplasma length L which is the length of discharge plasma, based on thesecondary voltage V2 and the minimum value V2 min of the secondaryvoltage. As seen from the concept of calculation of the discharge plasmalength mentioned above, the discharge plasma length L can be calculatedbased on the secondary voltage V2 and the minimum value V2 min of thesecondary voltage.

In the present embodiment, the discharge plasma length calculation unit54 calculates the discharge plasma length L using the equation (13). Thedischarge plasma length calculation unit 54 calculates the dischargeplasma length L at each time during the discharge period using theequation (13). Here, the discharge plasma length calculation unit 54calculates the discharge plasma length L(m) of each sampling number mcorresponding to each time during the discharge period, based on theminimum value V2 min of the secondary voltage, and the secondary voltageV2(m) of each sampling number m, by use of the next equation obtained bymodifying the equation (13) in accordance with the present embodiment.That is, the discharge plasma length calculation unit 54 changes thesampling number m from the A/D conversion start number to the endnumber, and repeatedly performs calculation of the next equation; and asshown in FIG. 16, stores the each discharge plasma length L (m) to thestorage apparatus 91 such as RAM by correlating with the sampling numberm.

$\begin{matrix}{{L(m)} = {{Lg} \cdot \left( \frac{V\; 2(m)}{V\; 2\;\min} \right)^{n}}} & (27)\end{matrix}$

Here, the plug gap length Lg is preliminarily set to a fixed value. Theexponent n is set to a value within a range of 1.0 to 3.0. For example,as the simplest approximation, the exponent n is preliminarily set tothe fixed value of 2.0 (n=2). By this setting, the computation load ofthe controller 50 can be reduced.

Alternatively, as mentioned above, since the exponent n changesaccording to the cylinder internal pressure and the like at the ignitiontiming, the discharge plasma length calculation unit 54 may change theexponent n within the range of 1.0 to 3.0 according to the drivingcondition of the internal combustion engine correlated with the cylinderinternal pressure at the ignition timing. For example, the dischargeplasma length calculation unit 54 calculates the exponent ncorresponding to the present driving condition, by referring to anexponent setting map in which the relationship between the exponent nand the driving condition correlated with the cylinder internal pressureis preliminarily set. As the driving condition correlated with thecylinder internal pressure at the ignition timing, the chargingefficiency may be used, and in addition to the charging efficiency, therotational speed of the internal combustion engine may be used.Alternatively, as the driving condition correlated with the cylinderinternal pressure at the ignition timing, an estimated cylinder internalpressure which is estimated by the charging efficiency, a relationalequation of polytope and the like may be used.

In the successive step 1204, the discharge plasma length calculationunit 54 calculates a maximum value Lmax of the discharge plasma length Lduring the discharge period based on the discharge plasma length Lduring the discharge period. For example, the discharge plasma lengthcalculation unit 54 changes the sampling number m from the A/Dconversion start number to the end number, and repeatedly performs aprocessing, as shown in the next equation, which picks up a maximumvalue between the discharge plasma length L (m) of the sampling number mand the maximum value Lmax of the discharge plasma length in theprevious processing and updates the picked up value as the maximum valueLmax of the discharge plasma length in this time processing; andconsequently, calculates the maximum value Lmax of the discharge plasmalength during the discharge period. Here, max( ) represents theprocessing which picks up the maximum value. The maximum value Lmax ofthe discharge plasma length is reset to 0 every ignition.Lmax=max(L(m),Lmax)  (28)<Calculation of Blow Off Number Nr>

In the successive step 1205, the discharge plasma length calculationunit 54 calculates a blow off number Nr of the discharge plasma duringthe discharge period based on the discharge plasma length L during thedischarge period. The blow off of the discharge plasma means that thedischarge plasma is broken off because the in-cylinder flow is toostrong; and when energy remains in the core of the ignition coil afterthe blow off, the spark discharge (re-discharge) usually occurs again.For this reason, in the case where the in-cylinder flow is strong, theblow off of the discharge plasma may occur several times during oneignition. There is a path shortening of the discharge plasma as asimilar phenomenon. The path shortening means that when the dischargeplasma extended, the discharge plasma switches to a short path becausethe discharge plasma extended long contacts as it got entangled.

FIG. 17 is a timing chart showing the behavior of the secondary coil 132side in the case where the blow off and the path shortening of thedischarge plasma occur. At the time t3 of FIG. 17, after the blow off ofthe discharge plasma, the re-discharge occurs. The discharge plasmalength L just after re-discharge starting becomes short again to theplug gap length Lg. At the time t3, the secondary voltage V2 (absolutevalue) rises to near the breakdown voltage Vbk in a short time, after itcauses the dielectric breakdown, by the start of re-discharge, thesecondary voltage V2 falls to the minimum value V2 min. At the time t3,after the secondary current I2 (absolute value) falls to zero by theblow off of the discharge plasma, by the start of re-discharge, thesecondary current I2 recovers to the same level as just before the blowoff according to the magnetic energy.

At the time t4, the path shortening occurs. The discharge plasma lengthjust after the path shortening becomes short again to near the plug gaplength Lg. At the time t4, although the secondary voltage V2 falls tothe minimum value V2 min, it is no necessary to cause the dielectricbreakdown, and the secondary voltage V2 does not rise. At the time t4, avariation like a noise is superimposed on the secondary current I2. Itis considered that the blow off and the path shortening of the dischargeplasma influence flammability similarly and they are counted to the blowoff number Nr here. But, only the blow off may be counted.

When the blow off and the path shortening of the discharge plasma occur,the discharge plasma length becomes short instantly. At this time, sincethe secondary voltage V2 falls to near the minimum value V2 min, thedischarge plasma length L calculated by the equation (13) or theequation (27) becomes small instantly. Accordingly, in the case wherethe time decrease amount ΔL of the discharge plasma length L is largerthan a preliminarily set blow off determination value Kjdg, thedischarge plasma length calculation unit 54 determines that the blow offoccurred and makes the blow off number Nr increase by one.

The discharge plasma length calculation unit 54 calculates a timedecrease amount ΔL by subtracting the discharge plasma length L (m) ofthe present sampling number (m) from the discharge plasma length L (m−1)of the previous sampling number (m−1), as shown in the next equation;determines whether or not the time decrease amount ΔL is larger than theblow off determination value Kjdg; and in the case where it is larger,increases the blow off number Nr by one. The discharge plasma lengthcalculation unit 54 changes the sampling number m from the A/Dconversion start number to the end number, performs these processingsrepeatedly, and calculates the blow off number Nr during the dischargeperiod. Here, the blow off determination value Kjdg is preferably set toa value within a range from the same as the plug gap length Lg toseveral times of the plug gap length Lg. The blow off number Nr is resetto 0 every ignition.ΔL(m)=L(m−1)−L(m)ΔL(m)>Kjdg  1)Nr=Nr+1ΔL(m)≤Kjdg  2)Nr=Nr  (29)

If the path shortening is excluded from count of the blow off number Nr,only when the secondary voltage V2 rises to the value near the breakdownvoltage Vbk in addition to the condition that the time decrease amountΔL of the discharge plasma length L becomes larger than the blow offdetermination value Kjdg, the blow off number Nr may be increased byone.

By calculating the blow off number Nr in this way, in addition to thedischarge plasma length L, the value correlated with the in-cylinderflow can be calculated, and the determination accuracy of the strengthof the in-cylinder flow can be improved.

<In-Cylinder Flow Calculation Unit 58>

In the successive step 1206, the in-cylinder flow calculation unit 58performs an in-cylinder flow calculation processing (in-cylinder flowcalculation step) that calculates the in-cylinder flow speed va which isa flow speed of the gas in the combustion chamber, based on the timechange of the discharge plasma length L and the Coulomb force applied tothe charged particles of the discharge plasma. As seen from the conceptof in-cylinder flow speed calculation mentioned above, the in-cylinderflow speed va can be calculated based on the time change of thedischarge plasma length L and the Coulomb force of the discharge plasma.

In the present embodiment, the in-cylinder flow calculation unit 58calculates the moving speed vp at which the intermediate part 32 ofdischarge plasma moves in the in-cylinder flow direction by extension ofthe discharge plasma by the in-cylinder flow, based on the time changeof the discharge plasma length L. For example, the next equation similarto the equation (17) is used. The equation (17) is derived fromsupposing that the discharge plasma is modeled in the U shape, and twoparts of the first side part 30 and the second side part 31 of the Ushape extend at the moving speed vp of the intermediate part 32. Thein-cylinder flow calculation unit 58 calculates a value obtained bydividing the time change rate of the discharge plasma length L by 2, asthe moving speed vp of the intermediate part 32 of discharge plasma. Thein-cylinder flow calculation unit 58 calculates, as the time change rateof the discharge plasma length L, a value obtained by dividing a valueobtained by subtracting the discharge plasma length L (m−1) of theprevious sampling number (m−1) from the discharge plasma length L (m) ofthe present sampling number (m) by the sampling time interval Δt, andcalculates a value obtained by dividing this time change rate by 2, asthe moving speed vp (m) of the intermediate part 32 of the presentsampling number (m). As shown in FIG. 16, the in-cylinder flowcalculation unit 58 changes the sampling number m from the A/Dconversion start number to the end number, and repeatedly performscalculation of the moving speed vp and memorizes it to the storageapparatus 91 such as RAM.

$\begin{matrix}{{{vp}(m)} = {\frac{1}{2} \cdot \frac{{L(m)} - {L\left( {m - 1} \right)}}{\Delta\; t}}} & (30)\end{matrix}$

As mentioned above, to the discharge plasma as the electric tube offorce, the tension of Maxwell stress pm shrinking the discharge plasmain the direction along the discharge plasma is applied, and the pressureof Maxwell stress pm pressing the side face of the discharge plasma inthe direction perpendicular to the discharge plasma is applied. Thedischarge plasma behaves like the elastic body which received theelectric strain. Accordingly, Maxwell stress pm acts on the dischargeplasma which tends to extend according to the in-cylinder flow. Thein-cylinder flow calculation unit 58 calculates Maxwell stress pmapplied to the discharge plasma by the Coulomb force, based on thesecondary voltage V2 and the discharge plasma length L. The in-cylinderflow calculation unit 58 calculates Maxwell stress pm by using the nextequation similar to the equation (21) and the equation (22). Here, thedielectric constant of vacuum ε0 is a preliminarily set value. As shownin FIG. 16, the in-cylinder flow calculation unit 58 changes thesampling number m from the A/D conversion start number to the endnumber, and repeatedly performs calculation of the Maxwell stress pm andmemorizes it to the storage apparatus 91 such as RAM.

$\begin{matrix}{{{pm}(m)} = {{\frac{1}{2} \cdot ɛ}\;{0 \cdot \left( \frac{V\; 2(m)}{L(m)} \right)^{2}}}} & (31)\end{matrix}$

The in-cylinder flow calculation unit 58 calculates the in-cylinder flowspeed va, based on the moving speed vp of the intermediate part 32 ofthe discharge plasma and the Maxwell stress pm. According to thisconfiguration, by considering a deviation between the in-cylinder flowspeed va and the moving speed vp of the discharge plasma, which iscaused by the tension of Maxwell stress pm and the like acting on thedischarge plasma, the in-cylinder flow speed va can accurately becalculated.

As mentioned above, about the intermediate part 32 of the dischargeplasma modeled in the U shape, the equation (24) can be derived from theequation of momentum of the equation (23) considering Maxwell stress pm.The in-cylinder flow calculation unit 58 calculates the in-cylinder flowspeed va based on the moving speed vp of the intermediate part 32 of thedischarge plasma, and Maxwell stress pm, by using the first equation ofthe next equation similar to the equation (24).

$\begin{matrix}{{va} = \sqrt{{vp}^{2} + {K \cdot \frac{pm}{\rho}}}} & (32) \\{K = {\frac{\pi \cdot r}{Lg} - 1}} & \;\end{matrix}$

Here, the calculation coefficient K may be preliminarily set to a valuecalculated by the plug gap length Lg and the radius r of the dischargeplasma using the second equation of the equation (32), or, may be set toa matching value adjusted based on experimental data. The in-cylinderflow calculation unit 58 calculates a volume of the combustion chamber25 based on the crank angle, using a characteristic data that representsa geometrical relation between the crank angle and the volume of thecombustion chamber 25, and calculates the gas density ρ by dividing theintake air amount taken into in the combustion chamber 25 by the volumeof the combustion chamber 25. As shown in FIG. 16, the in-cylinder flowcalculation unit 58 changes the sampling number m from the A/Dconversion start number to the end number, and repeatedly performscalculation of the in-cylinder flow speed va and memorizes it to thestorage apparatus 91 such as RAM.

Alternatively, as seen from the equation (25) which approximated theequation (24), the in-cylinder flow speed va can be calculated by addinga value according to Maxwell stress pm to the moving speed vp of theintermediate part 32 of discharge plasma. Then, the in-cylinder flowcalculation unit 58 may calculate a value obtained by adding a valueaccording to Maxwell stress pm to the moving speed vp of theintermediate part 32 of discharge plasma, as the in-cylinder flow speedva. Here, the in-cylinder flow calculation unit 58 may calculate thevalue according to Maxwell stress pm by using the right-hand side secondterm of the equation (25), or may calculate the value according toMaxwell stress pm by using the other function which uses Maxwell stresspm and the moving speed vp of the intermediate part 32 of dischargeplasma as variables. For example, the function is set to an approximateexpression based on experimental data, and the like.

Next, processing which operates the in-cylinder flow based on thedischarge plasma length L, the in-cylinder flow speed va, and the likewill be explained, referring to the flowchart of FIG. 18.

<Flow Correlation Value Calculation Unit 55>

In the step 1501, the flow correlation value unit 55 implements a flowcorrelation value calculation processing (a flow correlation valuecalculation step) that calculates a flow correlation value Cv, whichrepresents a strength of an in-cylinder flow which is a flow in thecombustion chamber 25, based on one or both of the discharge plasmalength L and the in-cylinder flow speed va. In the case of using thedischarge plasma length L, the flow correlation value calculation unit55 calculates the flow correlation value Cv, based on the maximum valueLmax of the discharge plasma length during the discharge period which iscalculated based on the discharge plasma length L during the dischargeperiod. As shows in FIG. 19, the flow correlation value calculation unit55 makes the flow correlation value Cv increase gradually, as themaximum value Lmax of the discharge plasma length during the dischargeperiod increases from the preliminarily set plug gap length Lg.

In the case of using the in-cylinder flow speed va, the flow correlationvalue calculation unit 55 calculates the flow correlation value Cv basedon a statistical processing value, such as an average value or a maximumvalue, of the in-cylinder flow speed va during the discharge period. Asshown in FIG. 20, the flow correlation value calculation unit 55 makesthe flow correlation value Cv increase gradually as the statisticalprocessing value of the in-cylinder flow speed va increases.

The flow correlation value calculation unit 55 may use one of the flowcorrelation value Cv calculated using the discharge plasma length L, andthe flow correlation value Cv calculated using the in-cylinder flowspeed va, or may use an average value of both, or may switch which oneis used according to an operating condition.

When the blow off of the discharge plasma or the path shortening occursbecause the in-cylinder flow is too strong, the discharge plasma lengthL once becomes short by the blow off; for this reason, the maximum valueLmax of the discharge plasma length does not become as large as thestrength of the in-cylinder flow. When the blow off or the pathshortening occurs, the time change rate of the discharge plasma length Lis disrupted, and the calculation accuracy of the in-cylinder flow speedva is deteriorated. Accordingly, the flow correlation value calculationunit 55 calculates the flow correlation value Cv based on the maximumvalue Lmax of the discharge plasma length during the discharge period orthe in-cylinder flow speed va, and the blow off number Nr. Ifdeterioration of the calculation accuracy of the in-cylinder flow speedva can be suppressed by statistical processing of the in-cylinder flowspeed va, the blow off number Nr may not be considered in calculation ofthe flow correlation value Cv.

For example, in the case where the blow off number Nr during thedischarge period is 0 time, the flow correlation value calculation unit55 calculates the flow correlation value Cv based on the maximum valueLmax of the discharge plasma length during the discharge period or thein-cylinder flow speed va, as mentioned above. When the blow off numberNr during the discharge period is greater than or equal to once, theflow correlation value calculation unit 55 calculates the flowcorrelation value Cv based on the blow off number Nr. Here, the flowcorrelation value Cv based on the blow off number Nr is set to a largervalue than a strong flow determination value Ths described below. Thatis to say, when the blow off number Nr during the discharge period isgreater than or equal to once, the flow control unit 56 described belowdetermines that the in-cylinder flow is strong. As shown in FIG. 19 andFIG. 20, the flow correlation value calculation unit 55 makes the flowcorrelation value Cv increase gradually as the blow off number Nr duringthe discharge period increases.

<Flow Control Unit 56 and Ignition Energy Increase Unit 57>

In the step 1502 to the step 1505, the flow control unit 56 implements aflow control processing (a flow control step) that determines whetherthe in-cylinder flow is strong or weak based on the flow correlationvalue Cv; in the case of determining that the in-cylinder flow isstrong, controls a flow operation mechanism which can operate thein-cylinder flow, to a side which the flow is weakened; and in the caseof determining that the in-cylinder flow is weak, controls the flowoperation mechanism to a side which the flow is strengthened.

In the present embodiment, in the step 1502, the flow control unit 56determines whether or not the flow correlation value Cv is larger thanthe preliminarily set strong flow determination value Ths and thein-cylinder flow is strong. In the case of determining that the flowcorrelation value Cv is larger than the strong flow determination valueThs and the in-cylinder flow is strong in the step 1502, the flowcontrol unit 56 controls the flow operation mechanism to the side whichthe flow is weakened in the step 1503.

In the present embodiment, the flow operation mechanism is the variablevalve timing mechanism which can change the opening and closing timingof the intake valve 10 and the exhaust valve 11; and in the step 1503,the flow control unit 56 changes the phase angle of the opening andclosing timing of the intake valve 10, and the phase angle of theopening and closing timing of the exhaust valve 11 in the direction inwhich the in-cylinder flow is weakened. The flow control unit 56calculates the phase angles of the intake valve and the exhaust valvefor weakening the in-cylinder flow corresponding to the present drivingcondition, by referring to a phase angle map in which the relationshipbetween the driving condition, such as the rotational speed and thecharging efficiency, and the phase angles of the intake valve and theexhaust valve for weakening the in-cylinder flow is preliminarily set.Then, the flow control unit 56 changes the phase angles of the intakevalve and the exhaust valve toward the phase angles of the intake valveand the exhaust valve for weakening the in-cylinder flow.

In the step 1503, in the case of determining that the in-cylinder flowis strong, the ignition energy increase unit 57 commands the ignitioncoil control unit 51 to increase the ignition energy supplied to theignition plug 12. Specifically, the ignition coil control unit 51commands the ignition energy increase unit 57 to make the energizingtime increase from a value calculated according to driving condition. Ifthe energizing time is extended, because the magnetic energy stored inthe core 136 by the primary coil 131 increases, the secondary current I2becomes large. In this case, since the cross-section area S of thedischarge plasma becomes large as thought by the equation (4), thedischarge plasma becomes difficult to blow off and the ignitability offuel-air mixture is improved.

On the other hand, in the case of determining that the flow correlationvalue Cv is not larger than the strong flow determination value Ths inthe step 1502, the flow control unit 56 advances to the step 1504, anddetermines whether or not the flow correlation value Cv is smaller thana weak flow determination value Thw which is preliminarily set to avalue less than or equal to the strong flow determination value Ths, andthe in-cylinder flow is weak. In the case of determining that the flowcorrelation value Cv is smaller than the weak flow determination valueThw and the in-cylinder flow is weak in the step 1504, the flow controlunit 56 controls the flow operation mechanism (the phase angles of theintake valve and the exhaust valve) to the side which the flow isstrengthened in the step S1505.

In the present embodiment, in the step 1505, the flow control unit 56changes the phase angle of the opening and closing timing of the intakevalve 10 and the phase angle of the opening and closing timing of theexhaust valve 11 in the direction in which the in-cylinder flow isstrengthened. The flow control unit 56 calculates the phase angles ofthe intake valve and the exhaust valve for strengthening the in-cylinderflow corresponding to the present driving condition, by referring to aphase angle map in which the relationship between the driving condition,such as the rotational speed and the charging efficiency, and the phaseangles of the intake valve and the exhaust valve for strengthening thein-cylinder flow is preliminarily set. Then, the flow control unit 56changes the phase angles of the intake valve and the exhaust valvetoward the phase angles of the intake valve and the exhaust valve forstrengthening the in-cylinder flow.

In the step 1504, in the case of determining that the in-cylinder flowis weak, the ignition energy increase unit 57 does not change theignition energy supplied to the ignition plug 12. That is to say, theenergizing time is maintained at the value calculated according to thedriving condition.

On the other hand, in the case of determining that the flow correlationvalue Cv is not smaller than the weak flow determination value Thw inthe step 1504, the flow control unit 56 determines that it is in theintermediate flow state where the in-cylinder flow is neither strong norweak. Then, the flow control unit 56 does not change the flow operationmechanism (the phase angles of the intake valve and the exhaust valve)to the strengthening side or the weakening side, and then ends theprocessing. The ignition energy increase unit 57 does not change theignition energy, either.

As described above, by estimating the discharge plasma length L and thein-cylinder flow speed va, and controlling the in-cylinder flow and theignition energy based on one or both of the estimated discharge plasmalength L and the in-cylinder flow speed va, even in combustion such ashigh dilution combustion, which the stable combustion region is narrow,good flammability can be secured.

OTHER EMBODIMENTS

Lastly, other embodiments of the present disclosure will be explained.Each of the configurations of embodiments to be explained below is notlimited to be separately utilized but can be utilized in combinationwith the configurations of other embodiments as long as no discrepancyoccurs.

(1) In the above-mentioned Embodiment 1, there has been explained thecase where the secondary voltage detection unit 52 detects the secondaryvoltage V2 based on the output signal of the ignition coil voltagesensor 134 which detects the secondary voltage V2 directly. However,because the secondary voltage V2 is very high voltage, when this voltageis taken into the controller 50, a large noise is generated, and thereis a possibility that the controller 50 causes malfunction; so, a noisecountermeasure is required. Accordingly, the secondary voltage detectionunit 52 may detect a primary voltage V1 which is a voltage generated bythe primary coil 131, and detects the secondary voltage V2 based on theprimary voltage V1.

FIG. 21 shows a circuit configuration diagram of the spark plug 12 andthe ignition coil 13 in this case. The ignition coil 13 is provided withan ignition coil voltage sensor 138 which outputs an electric signalaccording to a primary voltage V1 which is a voltage generates by theprimary coil 131. The ignition coil voltage sensor 138 is a voltagedividing circuit which divides the primary voltage V1 by two resistancesconnected in series, and is connected in parallel with the igniter 133.The divided voltage of the connection point of two resistances isinputted to the controller 50.

One end of the primary coil 131 is connected to the positive electrodeof the direct current power source 20, and the other end of the primarycoil 131 is connected to the ground via the igniter 133. The other endof the primary coil 131 is connected to the ground via the ignition coilvoltage sensor 138 which is the voltage dividing circuit. One end of thesecondary coil 132 is connected to the positive electrode of the directcurrent power source 20, and the other end of the secondary coil 132 isconnected to the ground via the ignition plug 12.

The secondary voltage detection unit 52 performs an A/D conversion ofthe divided voltage of the voltage dividing circuit as the ignition coilvoltage sensor 138 using the A/D converter of the input circuit 92, anddetects the primary voltage V1 based on an A/D conversion value and avoltage dividing resistance ratio. Then, the secondary voltage detectionunit 52 detects a value obtained by multiplying a winding number ratio Nof the primary coil 131 and the secondary coil 132 to the detectedprimary voltage V1, as the secondary voltage V2, as shown in the nextequation. The winding number ratio N is a value obtained by dividing awinding number of the secondary coil 132 by a winding number of theprimary coil 131, and is preliminarily set.V2=V1·N  (33)

According to this configuration, since the primary voltage V1 whosevoltage is lower than the secondary voltage V2 is taken into thecontroller 50, noise can be suppressed and the noise countermeasure ofvoltage detection can be reduced as compared with Embodiment 1.

(2) In the above-mentioned Embodiment 1, there has been explained thecase where the flow operation mechanism is the variable valve timingmechanism which can change the opening and closing timing of the intakevalve 10 and the exhaust valve 11. However, embodiments of the presentdisclosure are not limited to the foregoing case. That is to say, theflow operation mechanism may be any mechanism, as long as it is amechanism which can operate the in-cylinder flow; one or more flowoperation mechanisms may be provided, and it may be controlled based onthe flow correlation value Cv. For example, the flow operation mechanismmay be an intake port valve which closes a part of the intake port andgenerates a swirl flow or a tumble flow in the combustion chamber 25.The controller 50 controls an electric actuator and changes an openingdegree of the intake port valve.

Generally, the intake port valve which generates the swirl flow iscalled a swirl control valve, for example, it closes only one side ofthe two intake ports, and can operate the strength of the swirl flow inthe combustion chamber 25. Generally, the intake port valve whichgenerates the tumble flow is called a tumble control valve, for example,it closes only upper side or lower side of the intake port, and canoperate the strength of the tumble flow in the combustion chamber 25.

In the case of determining that the flow is strong, the flow controlunit 56 changes the opening degree of the intake port valve to a sidewhere the flow is weakened (for example, the opening side); and in thecase of determining that the flow is weak, the flow control unit 56changes the opening degree of the intake port valve to the side whichthe flow is strengthened (for example, the closing side).

(3) In the above-mentioned Embodiment 1, there has been explained thecase where the discharge plasma length calculation unit 54 calculatesthe discharge plasma length L by the equation (13) or the equation (27).However, embodiments of the present disclosure are not limited to theforegoing case. That is to say, the discharge plasma length calculationunit 54 may use other methods, as long as it calculates the dischargeplasma length L based on the secondary voltage V2 and the minimum valueV2 min of secondary voltage. For example, the discharge plasma lengthcalculation unit 54 may calculate the discharge plasma length Lcorresponding to this time secondary voltage V2 and this time minimumvalue V2 min of secondary voltage, by referring to a plasma length mapin which the relationship between the secondary voltage V2 and theminimum value V2 min of secondary voltage (for example, V2/V2 min), andthe discharge plasma length L is preliminarily set.

(4) In the above-mentioned Embodiment 1, there has been explained thecase where in the case where the flow correlation value Cv is largerthan the strong flow determination value Ths, the flow control unit 56controls the flow operation mechanism to the side where the flow isweakened; in the case where the flow correlation value Cv is smallerthan the weak flow determination value Thw, the flow control unit 56controls the flow operation mechanism to the side where the flow isstrengthened. However, embodiments of the present disclosure are notlimited to the foregoing case. That is to say, the flow control unit 56may use any method, as long as it determines the strength of thein-cylinder flow based on the flow correlation value Cv, controls theflow operation mechanism to the side where the flow is weakened in thecase determining that the flow is strong, and controls the flowoperation mechanism to the side where the flow is strengthened in thecase of determining that the flow is weak. For example, the flow controlunit 56 may change an operating amount of the flow operation mechanismto the side where the flow is strengthened or weakened, according to adifference between a target flow correlation value and the flowcorrelation value Cv.

Various modifications and alterations of this disclosure will beapparent to those skilled in the art without departing from the scopeand spirit of this disclosure, and it should be understood that this isnot limited to the illustrative embodiments set forth herein.

What is claimed is:
 1. A controller for an internal combustion enginethat is provided with an ignition plug which has a plug gap disposed ina combustion chamber, and an ignition coil which has a primary coil inwhich power is supplied from a direct current power source and asecondary coil which has more winding number than the primary coil andgenerates high voltage supplied to the ignition plug, the controller forthe internal combustion engine comprising: an ignition coil controllerthat shuts down after connecting electrically the primary coil and thedirect current power source for generating high voltage in the secondarycoil and generating spark discharge in the plug gap; a secondary voltagedetector that detects a secondary voltage which is a voltage generatedby the secondary coil; a secondary voltage minimum value calculator thatcalculates a minimum value of the secondary voltage during a dischargeperiod based on the detected secondary voltage; a discharge plasmalength calculator that calculates a length of the discharge plasma basedon the secondary voltage and the minimum value of the secondary voltage;and an in-cylinder flow calculator that calculates an in-cylinder flowspeed which is a flow speed of gas in the combustion chamber, based on atime change of the length of the discharge plasma and a Coulomb forceapplied to charged particles of the discharge plasma.
 2. The controllerfor the internal combustion engine according to claim 1, wherein thein-cylinder flow calculator calculates a moving speed at which anintermediate part of the discharge plasma moves in an in-cylinder flowdirection by extension of the discharge plasma by an in-cylinder flow,based on the time change of the length of the discharge plasma;calculates a Maxwell stress applied to the discharge plasma by theCoulomb force, based on the secondary voltage and the length of thedischarge plasma; and calculates the in-cylinder flow speed, based onthe moving speed of the intermediate part of the discharge plasma andthe Maxwell stress.
 3. The controller for the internal combustion engineaccording to claim 2, wherein the in-cylinder flow calculator calculatesa value obtained by dividing a time change rate of the length of thedischarge plasma by 2, as the moving speed of the intermediate part ofthe discharge plasma; and by setting the Maxwell stress to pm, settingthe secondary voltage to V2, setting the length of the discharge plasmato L, and setting a preliminarily set dielectric constant of vacuum toε0, calculates the Maxwell stress by a calculation equation of“pm=1/2×ε0×(V2/L)²”.
 4. The controller for the internal combustionengine according to claim 2, wherein the in-cylinder flow calculatorcalculates a value obtained by adding a value according to the Maxwellstress to the moving speed of the intermediate part of the dischargeplasma, as the in-cylinder flow speed.
 5. The controller for theinternal combustion engine according to claim 2, wherein by setting thein-cylinder flow speed to va, setting the moving speed of theintermediate part of the discharge plasma to vp, setting the Maxwellstress to pm, setting a density of the gas in the combustion chamber toρ, and setting a preliminarily set calculation coefficient to K, thein-cylinder flow calculator calculates the in-cylinder flow speed by acalculation equation of “va=√(vp²+K×pm/ρ)”.
 6. The controller for theinternal combustion engine according to claim 1, wherein by setting thelength of the discharge plasma to L, setting the secondary voltage toV2, setting the minimum value of the secondary voltage to V20, setting apreliminarily set length of the plug gap to Lg, and setting an exponent,which is set to a value within a range of 1.0 to 3.0, to n, thedischarge plasma length calculator calculates the length of thedischarge plasma by a calculation equation of “L=Lg×(V2/V20)^(n)”. 7.The controller for the internal combustion engine according to claim 6,wherein the exponent is set to 2.0.
 8. The controller for the internalcombustion engine according to claim 6, wherein the discharge plasmalength calculator changes the exponent within the range of 1.0 to 3.0,according to a driving condition of the internal combustion enginecorrelated with a pressure in the combustion chamber at ignition timing.9. The controller for the internal combustion engine according to claim1, further comprising: a flow correlation value calculator thatcalculates a flow correlation value, which represents a strength of aflow in the combustion chamber, based on the in-cylinder flow speed; anda flow controller that determines whether the flow in the combustionchamber is strong or weak based on the flow correlation value; in thecase of determining that the flow is strong, controls a flow operationmechanism which can operate the flow of the combustion chamber, to aside where the flow is weakened; and in the case of determining that theflow is weak, controls the flow operation mechanism to a side where theflow is strengthened.
 10. The controller for the internal combustionengine according to claim 1, further comprising: a flow correlationvalue calculator that calculates a flow correlation value, whichrepresents a strength of a flow in the combustion chamber, based on thein-cylinder flow speed; and an ignition energy increase controller thatdetermines whether the flow in the combustion chamber is strong or weakbased on the flow correlation value; and in the case of determining thatthe flow is strong, commands the ignition coil controller to increase anignition energy supplied to the ignition plug.
 11. The controller forthe internal combustion engine according to claim 9, wherein thedischarge plasma length calculator calculates a blow off number which isa number of times that the discharge plasma broke off during thedischarge, based on the length of the discharge plasma during thedischarge period, and wherein the flow correlation value calculatorcalculates the flow correlation value based on the in-cylinder flowspeed and the blow off number.
 12. The controller for the internalcombustion engine according to claim 9, wherein the flow operationmechanism is one or both of a variable valve timing mechanism which canchange an opening and closing timing of an intake valve and an exhaustvalve, and an intake port valve which closes apart of an intake port andgenerates a swirl flow or a tumble flow in the combustion chamber. 13.The controller for the internal combustion engine according to claim 1,wherein the secondary voltage detector detects a voltage generated bythe primary coil, and detects a value obtained by multiplying a windingnumber ratio of the primary coil and the secondary coil to the voltageof the primary coil, as the secondary voltage.
 14. A control method foran internal combustion engine that is provided with an ignition plugwhich has a plug gap disposed in a combustion chamber, and an ignitioncoil which has a primary coil in which power is supplied from a directcurrent power source and a secondary coil which has more winding numberthan the primary coil and generates high voltage supplied to theignition plug, the control method for the internal combustion enginecomprising: shutting down after connecting electrically the primary coiland the direct current power source for generating high voltage in thesecondary coil and generating spark discharge in the plug gap; detectinga secondary voltage which is a voltage generated by the secondary coil;calculating a minimum value of the secondary voltage during a dischargeperiod based on the detected secondary voltage; calculating a length ofdischarge plasma based on the secondary voltage and the minimum value ofthe secondary voltage; and calculating an in-cylinder flow speed whichis a flow speed of gas in the combustion chamber, based on a time changeof the length of the discharge plasma and a Coulomb force applied tocharged particles of the discharge plasma.